Facebook joined the $100 billion club, and just in time for Mark Zuckerberg (left) to cash in — if he wants.

The social network’s shares continued to climb yesterday, closing at $41.31, pushing the company’s market cap above $100 billion.

Zuckerberg’s company went public in May 2012 at $38 a share, but fell dramatically for the better part of the year following the high-profile initial public offering.

In September 2012, Facebook’s stock was below $18, prompting Zuckerberg to declare he would not sell any of his shares for a year, a move meant to calm investors and stabilize the share price, which was being hurt by the flood of insiders who were able to sell when lockup periods expired.

Now, the CEO will be free to sell shares starting in September, and his stock fortune has never been higher, with his stake valued at $20.8 billion, based on yesterday’s closing price.

Wall Street has finally started to fall in love with Facebook, after the rocky start. The company had been fending off worried shareholders, who as recently as the Facebook annual meeting in June were hammering Zuckerberg over the lagging stock price.

Then second-quarter earnings showed just how fast Facebook’s mobile advertising business was growing, beating analyst expectations.

The stock hasn’t looked back, last week closing above $40 for the first time, and yesterday topping the $100 billion valuation, a milestone many had thought would come with a pop on Facebook’s debut trading day.

Ken Sena, of Evercore Partners, was among the prognosticators who were made to look bad by Facebook’s flameout, and his early target price of $50 a share was named “worst prediction of 2012.”

Sena’s earlier call appears within reach, but he currently has a $45 target on shares.

“Improvements in ad targeting, attractive ad pricing and stable-to-growing user engagement, lead us to see room for meaningfully higher revenue-per-user,” Sena wrote in his latest note to clients.

Facebook has more than 1 billion users, and 700 million of them access the social network from smartphones and tablets. At the end of the last quarter, 41 percent of ad revenue came from mobile.

The company continues to hone its advertising offerings, filling mobile news feeds with sponsored content, and it is experimenting with video ads.